This invention relates to a measuring device for measuring the magnitude of deviation such as movement, rotation or the like carried out between two objects, and more particularly to a measuring device generating an output proportional to a velocity of deviation.
In a machine tool or the like, it is conventionally known that it is critical in precision working to precisely measure movement of a tool in relation to a workpiece. For this purpose, various measuring devices have been developed and manufactured as a commercial product.
One of such conventional measuring devices is an optical scale utilizing a moire fringe obtained by superposing two optical lattices on each other. An optical scale which has been conventionally used for this purpose is typically constructed in such a manner as shown in FIGS. 6(a) and 6(b). More particularly, it includes a main scale 101 constituted by a lattice provided on a glass plate in such a manner that light-permeable sections and light-impermeable sections are arranged at predetermined intervals and an index scale 102 constituted by a lattice provided on a glass plate in such a manner that light-permeable sections and light-impermeable sections are arranged at predetermined intervals. The index scale 102 is arranged so as to be spaced at a microinterval from the main scale 101 and opposite to the main scale 101 while being kept inclined by a microangle with respect to the main scale 101.
The lattice thus arranged on each of the main scale 101 and index scale 102 is made by forming a film of Cr on one surface of the glass plate by vacuum deposition and then etching the Cr film to form indented lines at equal pitches on the glass plate.
Arrangement of the main scale 101 and index scale 102 in a manner to be opposite to each other at a microinterval causes occurrence of a moire fringe as shown in FIG. 8. The moire fringe has a cycle of W, resulting in producing darkness or brightness at every cycle W. Such darkness or brightness are downwardly or upwardly moved depending on a direction in which the index scale 102 is laterally moved relatively to the main scale
When a pitch of the lattice of each of the main scale 101 and index scale 102 is indicated at P and an angle of inclination of the index scale 102 with respect to the main scale 101 is indicated at .theta. (tad), the cycle W of the moire fringe is represented by the following expression: EQU w=P/.theta.
Thus, the cycle W of the moire fringe corresponds to a cycle obtained by optically magnifying the lattice pitch P by a factor of 1/.theta.. Therefore, movement of the lattice by P causes movement of the moire fringe by W, so that reading of a variation of W magnified permits the magnitude of movement of the lattice to be precisely measured.
In view of the above, in order to optically detect a variation of the moire fringe, a light emitting element 103 and a light receiving element 105 are arranged on a head body 106 provided with the index scale 102, as shown in FIGS. 7(a) and 7(b).
More particularly, as shown in FIG. 7(b), the head body 106 is provided on a side thereof opposite to the main scale 101 with the light emitting element (light source) 103, to thereby permit the light receiving element (photo-electric transfer element) 105 to receive light of the light emitting element 103 which has permeated the moire fringe, resulting in a variation of the moire fringe being optically detected.
Reading of a variation in current flowing to the photoelectric transfer element 105 while moving the index scale 102 relatively to the main scale 101 indicates that the current is varied in a manner like a sine wave as shown in FIG. 9.
Thus, arrangement of two or A-phase and B-phase photoelectric transfer elements 107 and 108 while deviating them from each other by a sum of one cycle W (360.degree.) and 90.degree. causes a current flowing to the A-phase photoelectric transfer element 107 to have a waveform like a sine wave, as well as a current flowing to the B-phase photoelectric transfer element 108 to have a waveform like a cosine wave.
In this instance, a phase of a current flowing to the B-phase photoelectric transfer element 108 is advanced or delayed by 90.degree. with respect to that of a current flowing to the A-phase photoelectric transfer element 107 depending on a direction of relative movement between the main scale 101 and the index scale 102, so that arrangement of the two photoelectric transfer elements while deviating them from each other by 90.degree. permits deviation between both phases to be detected, resulting in a direction of relative movement therebetween being detected.
One cycle P of each of both A-phase signal and B-phase signal corresponds to movement of the index scale 102 by the pitch P of the lattice, so that counting of the A-phase signal and B-phase signal by a counter after waveform shaping of the signals permits a distance of movement of the index scale 102 to be measured.
In the optical scale described above, precise formation of the lattice on the glass plate requires that a pitch of the lattice is defined to be microns to tens of microns or more. Unfortunately, this fails to permit the optical scale to measure a distance of the movement as small as a sub-micron.
In order to solve the problem, an interpolation circuit is used which is adapted to divide the pitch of the lattice for interpolation, to thereby permit a distance of the movement below a sub-micron to be measured.
Such an interpolation circuit may be constructed in such a manner as shown in FIG. 10 by way of example.
In the interpolation circuit shown in FIG. 10, a sine wave 111 and a cosine wave 112 inputted to a voltage adding-subtracting circuit 117 are the same as the A-phase signal and B-phase signal shown in FIG. 9 and outputted in the form of a sin .theta. signal and a con .theta. signal from the voltage adding-subtracting circuit 117, respectively. Also, the voltage adding-subtracting circuit 117 adds the sine wave 111 and cosine wave 112 to each other to output a sin(.theta.+45.degree.) signal therefrom and subtracts the cosine wave 112 from the sine wave 111 to output a sin(.theta.-45.degree.) signal therefrom.
The four signals thus outputted from the voltage adding-subtracting circuit 117 are then inputted to a waveform shaping circuit 118, wherein the signals each are shaped into a pulse waveform, resulting in pulse signals indicated at a, b, c and d in FIG. 11 being provided.
In this instance, one cycle P of each of the sine wave 111 and cosine wave 112 constitutes a cycle P shown in FIG. 11. Then, the pulse signals a and b each are inputted to an exclusive OR circuit (EX-OR) 119, resulting in a pulse signal of which a frequency is doubled as indicated at an A phase in FIG. 11. Likewise, feed of each of the pulse signals c and d to an exclusive OR circuit (EX-OR) 120 provides a pulse signal of which a frequency is doubled as indicated at a B phase in FIG. 11.
Thus, the interpolation circuit outputs the pulse signals of the A and B phases of which a frequency is doubled. When a counter is to carry out counting each of the pulse signals at a leading edge of the signal and a trailing edge thereof, eight countings can be accomplished in one cycle P. Thus, interpolation of the pulse signal is increased by a factor of 8. Therefore, supposing that the pitch P of the lattice is 16 microns, a distance of the movement can be measured at a resolution of 2 microns.
Now, another conventional interpolation circuit, which is disclosed in Japanese Patent Application Laid-Open Publication No. 132104/1988, will be described with reference to FIG. 12.
In the interpolation circuit of FIG. 12, a sine wave 111 and a cosine wave 112 inputted to a balanced modulation adding circuit 113 are the A-phase signal and B-phase signal shown in FIG. 9, respectively. The balanced modulation adding circuit 113 subjects a sin .omega.t signal and a cos .omega.t signal fed in the form of a carrier from a digital circuit 116 to balanced modulation by means of the signals thus inputted, to thereby add the balanced modulated waves to the signals, followed by outputting of the signals. The output of the balanced modulation adding circuit 113 is then passed through a low-pass filter (LPF) 114, which extracts a sin(.omega.t-.theta.) signal component from the output and then feeds it to the waveform shaping circuit 115, resulting in it being shaped into a pulse signal.
The pulse signal is then fed to the digital circuit 116, so that the A-phase signal and B-phase signal interpolated are output therefrom.
The pulse signal a outputted from the waveform shaping circuit 115 has a phase subjected to phase modulation by the sine wave (sin .theta.) 111 and cosine wave (cos .theta.) 112 inputted thereto. More particularly, the pulse signal a has a width varied depending the magnitude of deviation of the index scale 102 and a direction of the deviation. Thus, as indicated at a in FIG. 13, the movement in a right-hand direction causes a pulse width of the pulse signal a to be increased, whereas the movement in a left-hand direction causes the width to be decreased.
A pulse width deviated from a reference width is outputted in the form of the A-phase signal (pulse width: t1) or the B-phase signal (pulse width: t2). The number of clocks shown in FIG. 13 which are within a pulse width of each of the A-phase and B-phase signals is counted, resulting in a value interpolated being obtained from the counter.
The carrier sin .omega.t and cos .omega.t signals are prepared by dividing frequencies of clocks counted by the counter. In this instance, a resolution of the interpolation circuit is determined by the number of dividing. More specifically, supposing that the number of dividing is 40, interpolation of 40 pulses is possible. Thus, when the pitch of the lattice is 40 microns, the scale exhibits a resolution of 1 micron.
However, the conventional interpolation circuit described above with reference to FIG. 10 fails to increase the number of interpolation (the number of dividing) due to an error of a resistance value in the voltage adding-subtracting circuit 117 and an error of offset of the waveform shaping circuit 115, resulting in failing to reducing a resolution.
Also, although the interpolation circuit shown in FIG. 12 improves a resolution by increasing the number of dividing, it fails to provide a velocity proportional output, to thereby fail to direct the output to other applications.